This invention relates to an Al-base brazing alloy which is satisfactory both in resistance to corrosion and mechanical properties.
Aluminum is well known for its good resistance to corrosion. However, in a brazed structure, for instance, in a heat exchanger, Si in the brazing material penetrates into core aluminum alloy to form local cells in the aluminum alloy which surrounds the cores of Si, giving rise to intergranular corrosion. In order to prevent such corrosion, there have been proposed various methods, although none of which succeeded in providing practical measures the preventing the intergranular corrosion. The conventionally proposed methods include:
1. Methods using anodically acting elements (e.g., Zn, Sn, etc.):
1-a. A method of imparting anodic property to the brazing alloy itself by adding thereto Zn, Sn or the like. PA1 1-b. A method of using a Zn- or Sn- material in part of brazed structure, e.g., fins of heat exchanger, for protecting other part of the structure by the anodic action of the added elements.
2. Methods of improving and strengthening the intergranular penetration resistance of core aluminum alloy to Si.
3. Method for reforming the core alloy to have cathodic action relative to the brazing material by adding a third component to the core alloy.
Of the above-mentioned methods 1a, the one which employs Zn remarkably improves the resistance to corrosion but has a difficulty in that the brazing furnace is contaminated with vapors of Zn which is spattered due to high vapor pressure at the time of brazing operation. The method which uses Sn instead of Zn is free from the problem of the evaporative spattering but has other problems such as low formability and difficulty of rolling which are detrimental to practical applications. The method 1-b can provide only incomplete protection since it is difficult to protect a structural part like the header tank of a heat exchanger which is not in direct contact with the fins. In the methods 2 and 3, attempts are made to improve the brazing properties, for example, by adding Cu, Fe, Cr, Zr or the like to an aluminum alloy (e.g., alluminum alloy of JIS 3003) which is widely used as a brazing sheet core alloy or by changing conditions of a heat treatment such as soaking, but in any case with unsatisfactory results. The above-mentioned alloy JIS 3003 which contains Mn as one of its major components is well known to have an extremely high resistance to corrosion but needs further improvement in properties since it becomes susceptible to intergranular corrosion upon heating for brazing (i.e., heating at high temperature).